The invention relates to moisture-curing polyisocyanate mixtures, to a process for their preparation and to their use as binders in lacquers, coatings, adhesives and sealing materials.
Moisture-curing prepolymers as a constituent of one-component lacquers have been known for a long time (e.g. H. Kittel, Lehrbuch der Lacke and Beschichtungen, 2, S. Hirzel Verlag, Stuttgart, Leipzig, 1998). Such systems consist of polyurethane prepolymers having free, terminal isocyanate groups (NCO groups). After application, crosslinking takes place under the influence of atmospheric moisture to form urea groups.
For quick drying, prepolymers based on aromatic isocyanates, in particular 2,4- and optionally 2,6-diisocyanatotoluene (TDI), are particularly suitable. In particular the isocyanurate-group-containing polyisocyanates based on the mentioned TDI types are valuable components for the preparation of quick-drying polyurethane coatings for the wood and furniture sector. Quick drying is caused by the high degree of crosslinking of these products. This leads to a rapid rise in the glass transition temperature during the crosslinking reaction to form urea groups.
However, because of the high degree of crosslinking, such quick-drying systems are often brittle and films produced from such lacquers do not have the required elasticity. Cracks can thus form in the lacquer, especially in the case of pronounced changes in temperature. This is true in particular of coatings on substrates that are subject to considerable variations in volume, for example wood.
In order to render such TDI trimers flexible, high molecular weight polyethers, for example, are suitable for synthesising the prepolymer. As a result of being rendered flexible, the polymers generally become so soft that the drying speed falls markedly. In practice, this means that such moisture-curing coatings based on polyurethane prepolymers dry markedly more slowly than, for example, the widely used nitrocellulose combi lacquers (NC lacquers). The compatibility of long-chained polyethers with TDI isocyanurates is limited, which occasionally leads, even with small amounts of polyether, to products which are cloudy or not stable to storage. The teaching of EP-A 1 582 543 represents a solution to the problems outlined above. In that specification it was found that elastic and at the same time quick-drying moisture-curing lacquers can be formulated if there is used as the isocyanate component a solvent-containing preparation comprising NCO-group-containing prepolymers based on TDI trimers and diphenylmethane diisocyanate, at least one of the prepolymers having been prepared using a polyether diol containing ethylene oxide blocks and having a number-average molecular weight of from 3000 to 4500 g/mol and an ethylene oxide content of from 2 to 18 wt. %, based on all the alkylene oxide units present in the diol.
A disadvantage of the coating systems based on EP-A 1 582 543 is the relatively low solids content of from 30 to 35 wt. % of the lacquers formulated therefrom, and in particular their poor resistance to light and weathering, which manifests itself in a negative manner as a strong tendency to yellowing.
Accordingly, it was an object of the present invention to provide polyisocyanate mixtures with which it is possible to produce lacquers and coatings which have comparable drying speeds compared with NC lacquers, exhibit the required degree of elasticity and high resistance to solvents and chemicals and do not have the disadvantages of the prior art outlined above.
In particular, lacquers based on the novel products are to have a solids content of >50 wt. % and are to dry equally as quickly. The tendency of the coatings to yellowing is to be improved markedly as compared with the prior art.
It has now been found that polyisocyanate mixtures based on prepolymers, based on TDI isocyanurates and specific nitrogen-containing polyethers, and aliphatic polyisocyanates can be formulated to moisture-curing lacquers which have a relatively high solids content of >50 wt. % and which can be processed to quick-drying, elastic coatings which at the same time have high hardness, very good resistance to solvents and chemicals and also good resistance to yellowing under the action of light.